fix(starters): update dependency prop-types to ^15.7.2 (gatsbyjs#11748)

This PR contains the following updates:

| Package | Type | Update | Change | References |
|---|---|---|---|---|
| prop-types | dependencies | patch | `^15.7.1` -> `^15.7.2` | [homepage](https://facebook.github.io/react/), [source](https://github.com/facebook/prop-types) |

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### Release Notes

<details>
<summary>facebook/prop-types</summary>

### [`v15.7.2`](https://github.com/facebook/prop-types/blob/master/CHANGELOG.md#&#8203;1572)

[Compare Source](https://github.com/facebook/prop-types/compare/v15.7.1...v15.7.2)

-   [Fix] ensure nullish values in `oneOf` do not crash ([#&gatsbyjs#8203;256](https://github.com/facebook/prop-types/issues/256))
-   [Fix] move `loose-envify` back to production deps, for browerify usage ([#&gatsbyjs#8203;203](https://github.com/facebook/prop-types/issues/203))

</details>

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starters/default/package.json

@@ -13,7 +13,7 @@
     "gatsby-plugin-sharp": "^2.0.20",
     "gatsby-source-filesystem": "^2.0.20",
     "gatsby-transformer-sharp": "^2.1.13",
-    "prop-types": "^15.7.1",
+    "prop-types": "^15.7.2",
     "react": "^16.8.1",
     "react-dom": "^16.8.1",
     "react-helmet": "^5.2.0"